Molded polyurethane screens having reinforcement therein are known in the art. However, in the past the dividing strips between the openings were relatively large, thereby causing the open area of the screen to be an undesirably low percentage of its surface, thereby in turn causing the screen to be relatively inefficient.
The present invention is an improvement over U.S. Pat. Nos. 4,819,809 and 4,857,176, both of which are expressly incorporated herein by reference hereto. The present invention provides improved screens with relatively high percentage open screening areas and high efficiencies.
The present invention also provides improved screens and screen deck assemblies that may be used in screening machines such as those described in U.S. Pat. Nos. 6,070,736, 8,113,358, 8,522,981, and U.S. Patent Pub. No. 2011/0036759, all of which are expressly incorporated herein by reference hereto. These screening machines, referred to as attrition screening devices, including for example, sifters, gyratory sifters, or graters, include a class of vibratory devices used to separate sized particles, as well as to separate solids from liquids. Sifters are used to screen, for example, minerals, feed material, plastic resins, and powders during industrial sorting and/or manufacturing operations.
Because sifters may be in continuous use, repair operations and associated downtimes need to be minimized as much as possible. Sifters currently include screening assemblies that have a plate or frame as a base and a wire mesh, cloth, or other perforated filter overlay positioned as a screen over the plate or frame. These filter screens often wear out over time due to the particular motion in the sifters, and subsequently require replacement. Screens currently being used in these sifters often wear out in three weeks or less. Also, woven wire cloth screens are problematic in that they can have inconsistent openings, sizes, or other irregularities due to inaccuracies in the weaving process.
Existing screens used in sifters are generally placed on a “ball tray” or “ball box” that captures a number of balls or other agitation producing members which repeatedly impact the screen assembly to dislodge particulate material that accumulates in the screen openings and thus helps de-blind the screens. Blinding is often a serious problem with woven wire cloth screens. The ball tray or box is cumbersome, has loose balls, and is often heavy. In some instances, the screens and/or ball trays or boxes in sifters have to be replaced at least twice a week. Further, the specific motion of the sifter may cause the ball trays or boxes to emit hazardous particles into the air that may then affect the health and safety of the operators of these sifting screening machines. Accordingly, a need exists for screens and screen deck assemblies to be used in these sifting screening machines that are safer, longer lasting, more easily removable and replaceable, lighter, provide more consistent and accurate opening sizes than existing screens, and reduces blinding problems without the use of multiple loose impact objects.